There is great interest in using single-walled carbon nanotubes (SWNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWNTs are extremely sensitive to the surrounding environmental changes [1-5, 14-18, 21-25, 30, 31, 35, 42-45, 50, 51]. To date, most research on SWNTs has focused on electronic devices, with relatively little work on optical biosensors. In order to use SWNTs as optic biosensors, some immediate questions needs to be solved such as how the sensors respond to chemical variables like pH [5c] and concentration of glucose, ethanol, various ions, or proteins.
SWNTs are a collection of semiconducting, metallic nanotubes and a mixture of them in different diameters that can be probed by various spectroscopic methods including Raman spectroscopy and UV/vis/NIR absorption spectroscopy. Raman spectroscopy can be used to determine many aspects of an SWNTs sample, including size distribution, disorder from defects or functionalization, and general electronic behaviors.
SWNTs possess unique optical properties as a result of their one-dimensional nature. Sharp peaks in the density of states, called van Hove singularities (VHS), arise from a quantization of the electronic wave vector in the 1-D system [26]. As a result of these singularities, SWNTs possess peaks in their optical spectra that correspond to interband transitions from the valence band to the conduction band. In addition, the transitions are found to be grouped in spectral space according to nanotube type (metallic vs. semiconducting) and band index, which are responsible for the observed sharp and pronounced optical absorption peaks in individual HiPco SWNTs [21, 23].
The side view of an SWNT 100 is illustrated in FIG. 1a. The SWNT has a first end 110, an opposite, second end 120 and a body portion defined therebetween the first end 110 and the second end 120. The body portion contains a carbon “wall” that is formed by a plurality of carbon atoms in certain arrangements as known to people skilled in the art. As illustrated in FIG. 1b, the SWNT 100 can be considered to have an exterior surface 130, an interior surface 140, and a cavity 150, respectively.
Because current techniques produce SWNTs in a mixture form with about one third of metallic nanotubes and two thirds of semiconducting nanotubes [46], separations of semiconducting SWNTs from metallic SWNTs are required for practical applications [6, 47]. The study of SWNT separations is a subject of intense exploration [18-20]. The discovery of surfactant-assisted dissolution of SWNTs in aqueous sodium dodecyl sulfate (SDS) solution [23] has greatly stimulated the progress in this exciting area [18-20].
Water-soluble SWNTs (ws-SWNTs) with undisrupted characteristic optical absorption features have been obtained by surface modifications such as functionalization with carboxylate groups [14] and surface coatings with surfactants [21, 23] or single stranded DNA [18, 19]. FIG. 1c illustrates an SWNT encased in polymeric material 170. It has been observed that the optical characteristics of surface modified SWNTs are pH sensitive [14, 18, 23-25], which suggests new opportunities for SWNTs based optical biosensor applications yet to be explored. Nanotubes may even be combined with recently developed nanolasers [33], nano waveguides [53] and nano optical fibers [34], to make optical nanosensors in the near future.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.